1. Field of the Invention
The present invention relates to a biaxially oriented polyester film, and more specifically, it relates to a biaxially oriented polyester film which is formed from modified polyethylene-2,6-naphthalenedicarboxylate, and is particularly useful as a base film for a magnetic recording tape and as an electrically insulating film.
2. Prior Art
A biaxially oriented polyethylene terephthalate film has been hitherto known as a base film for a magnetic recording tape. A magnetic recording tape formed of such a base film has a low coercive force. It is therefore necessary to decrease the tape thickness in order to make it possible to encase a longer tape in a cassette for recording of longer duration. However, when the tape thickness is decreased, there is a problem in that the running properties and durability of the tape deteriorate.
There are therefore a number of proposals for use of a biaxially oriented polyethylene-2,6-naphthalenedicarboxylate film having a high Young's modulus as a base film for a magnetic recording tape.
However, even if a magnetic recording tape is produced from the above polyethylene-2,6-naphthalenedicarboxylate film having a high Young's modulus and a low heat shrinkage factor, several problems occur when the thickness of the base film is decreased in order to achieve the long recording and reproduction.
For example, the tape undergoes elongation and deformation due to a varied tension at the time of start and stop in tape running, and due to this, a strain consequently occurs in recorded data. Further, such problems occur that one edge of the tape is elongated and bent, and the running tape is stuck in a guide.
Further, there might be another problem. Due to a reason that the affinity between various fine particles added for imparting lubricity and polyethylene-2,6-naphthalenedicarboxylate is insufficient, voids might be formed between the fine particles and the polymer, and the fine particles and peeled portions of the polymer might drop from the film. As a result, the base film is abraded at steps of die coater treatment and calender treatment in producing a magnetic tape.
On the other hand, a biaxially oriented polyethylenenaphthalate film is used as an electrically insulating film, and a biaxially oriented polyethylene-2,6-naphthalenedicarboxylate film is recently attracting attention. An electrically insulating film is required to have the following properties. (1) It is required to show no deterioration in mechanical properties, insulation performance and electric breakdown even when exposed to a high temperature for a long time. (2) It is required to cause no delamination (interlayer peeling) when used. (3) The amount of low-molecular-weight substances (oligomers) present in the film and on its film surface is required to be small.
A method of increasing the molecular orientation by increasing the draw ratio is effective for improving the heat deterioration resistance and decreasing the amount of oligomers exuded from the film surface, and this method is employed in general practice. However, this method involves another problem in that as a result of the increase in the area drawing ratio, the film tends to cause delamination. Therefore, the increasing of the draw ratio has a limit.
Oh the other hand, a decreasing of the area draw ratio might avoid the film delamination. However, in this case, the heat deterioration resistance decreases, and the amount of oligomers exuded from the film surface increases.